Late Neogene benthic foraminifera and stable carbon isotope record of the blake Outer Ridge

Carbon isotope and benthic foraminiferal data from Blake Outer Ridge, a sediment drift in the western North Atlantic (Ocean Drilling Program Sites 994 and 997, water depth ~ 2800 m), document variability in the relative volume of Southern Component (SCW) and Northern Component Waters (NCW) over the last 7 Ma. SCW was dominant before ~5.0 Ma, at ~3.6-2.4 Ma, and 1.2-0.8 Ma, whereas NCW dominated in the warm early Pliocene (5.0-3.6 Ma), and at 2.4-1.2 Ma. The relative volume of NCW and SCW fluctuated strongly over the last 0.8 Ma, with strong glacial-interglacial variability. The intensity of the Western Boundary Undercurrent was positively correlated to the relative volume of NCW. Values of Total Organic Carbon (TOC) were > 1.5% in sediments older than ~ 3.8 Ma, and not correlated to high primary productivity indicators, thus may reflect lateral transport of organic matter. TOC values decreased during the intensification of the Northern Hemisphere Glaciation (NHG, 3.8-1.8 Ma). Benthic foraminiferal assemblages underwent major changes when the sites were dominantly under SCW (3.6-2.4 and 1.2-0.8 Ma), coeval with the 'Last Global Extinction' of elongate, cylindrical deep-sea benthic foraminifera, which has been linked to cooling, increased ventilation and changes in the efficiency of the biological pump. These benthic foraminiferal turnovers were neither directly associated with changes in dominant bottom water mass nor with changes in productivity, but occurred during global cooling and increased ventilation of deep waters associated with the intensification of the NHG.

Upper Eocene to Oligocene strontium, carbon and oxygen isotope record of DSDP holes

We improved upper Eocene to Oligocene deep-sea chronostratigraphic control by integrating isotope (87Sr/86Sr, delta18O, delta13C) stratigraphy and magnetostratigraphy. Most previous attempts to establish the timing of isotope fluctuations have relied upon biostratigraphic age estimates which have uncertainties of 0.5 to over 4.0 m.y. Deep Sea Drilling Project (DSDP) Site 522 contains the best available upper Eocene to Oligocene magnetostratigraphic record which allows first-order correlations of isotope records (87Sr/86Sr, delta18O, delta13C) to the Geomagnetic Polarity Time Scale (GPTS). Empirical calibrations between the 87Sr/86Sr of foraminifera and magnetochronology at Site 522 allow more precise correlation of ,unknown' samples with the GPTS. For example, shallow water and high-latitude sections may be tied into the deep-sea record. Sr-isotope stratigraphic resolution for the latest Eocene to Oligocene is approximately 2 m.y.

Oxygen and carbon isotopic composition of benthic and planktonic foraminifers at DSDP Site 610

A record of carbon and oxygen isotopes in benthic and planktonic foraminifers has been obtained from the interval corresponding to the last 2.4 m.y. of Site 610, Holes 610 and 610A, with a sample resolution of about 30 kyr. The record from the late Quaternary (<0.9 Ma) shows large amplitudes and high frequencies in oxygen isotopic variation. Prior to 0.9 Ma the isotopic variability record is reduced in amplitude (but not in frequency) compared with the late Quaternary, suggesting lower ice-volume and climatic fluctuations, and higher average eustatic sea level. Left-coiling (L, polar) Neogloboquadrinapachyderma were not found in samples between 1.0 and 2.2 Ma, indicating less influence of polar front migrations in the Northeast Atlantic. Both polar planktonic faunas and larger isotope fluctuations reappear in the lowermost samples (2.3 to 2.4 Ma), pointing toward a period of larger climatic variability in the late Pliocene than in the early Quaternary. The variation in benthic d13C and hence in deep-water d13C seems to have been constant through the analyzed section, reflecting a stable variability in the production of North Atlantic Deep Water (NADW) and possibly in Norwegian-Greenland Sea Overflow. Preliminary analyses of amino-acid epimerization in N. pachyderma (L) indicate a constant rate of epimerization to approximately 0.3 Ma. Beneath this level the average epimerization rate is much reduced.